Purification of cellulose



This invention relates to the purification of crude cellulose obtained by an alkaline pulping process.

aired States Patent In alkaline pulping processes the wood, straw or other raw material is boiled under pressurewith a solution of sodium hydroxide with-or without some sodium sulphide. If sodium sulphide is not used, the method is termed the soda process; if it is used, the method is termed the I sulphate process. The immediate products of the soda and sulphate processes, which still contain some lignin and also a considerable proportion of pentosans and are dark in colour, are quite unsuitable for conversion into cellulose derivatives such as cellulose acetate of standard commercial qualities. Moreover, while the lignin con tent and colour can be considerably reduced by bleaching, e.g. with chlorine or hypochlorite, it is very difficult to reduce the pentosan content to anything approaching asatisfactory level, even by purification methods which L are very effective when applied to sulphite pulps. In consequence purified cellulose to be used in the manufacture of cellulose derivatives is at present rarely or never derived from crude cellulose obtained by an alkaline pulping process.

It is an object of the present inventionto provide a method of purifying soda and sulphate pulps such that the purified cellulose can be used in the manufacture of Y cellulose derivatives of goodquality.

In accordancewith the inventioncrude cellulose which has been obtained from lignocellulosic material such as wood or straw by an alkaline pulping process is'purified by heating it with an aqueous solution of an inorganic sulphite or bisulphite with or without free sulphur dioxide, bleaching the cellulose, and removing pentosans from the bleached cellulose by means of an aqueous solution of an alkali of concentration above 8% and preferably above 10%. For the sake of brevity, the treatment with the sulphite or bisulphite solution will be referred to simply as a sulphite boil.

The crude cellulose pulps are preferably heated with the sulphite or bisulphite solution under pressure to a temperature above 100 C. and especially above 120 C., e.g. between about 125 and 170 C. and especially about 130-l50 C. The treating agent is preferably a sulphite or bisulphite of an alkali metal or ammonium, an alkaline earth metal, or magnesium, and is best employed in concentration below 2%, preferably between about 0.25 and 1%. The solution may contain more than one salt, for example a mixture of an alkali metal sulphite and the corresponding bisulphite; moreover it may contain a small proportion, preferably not more than 0.25%, of free sulphur dioxide. The amount of the solution employed may be about 5-15 times the weight of the crude pulp, and it is usually sufficient to keep the solution at the treatment temperature for about 3090 minutes.

The sulphite boil may if desired be followed immediately by a treatment with alkali, especially a treatment with a caustic alkali solution of concentration below 3%, e.g. 0.l-2%, at about 100 C. or higher. We prefer however to omit this alkali treatment and to subject the pulp resulting, from the sulphite boil, preferably after washing it with hot or cold water, directly to a bleach.

The pulp is preferably bleached in two or more stages, any or all of which may be under acid or neutral conditions. We have found :it particularly effective to give the pulp either a two-stage bleach with substantially neutral hypochlorite solution, or a three-stage bleach in which the first two stages are effected with hypochlorite under acid conditions and the final 'stage' with substantially neutral hypochlorite. Each bleaching stage, whether under acid or neutral conditions, is preferably followed by a treatment with alkali 'toiremove-alkali-soluble substances formed by the bleach; for example the pulp may be heated to about '60100'-=C. with a causticsod'a solution of concentration below 2% and preferably 0.254%. While the two methods described give very similar results, the three-stage bleach tends to :give a higher yield of cellulose 'of somewhat higher viscosity; on the other hand, the clarity of cellulose acetatemade from cellulose that has been given the two-stage bleach is in many instances slightly the better.

By means of the sulphite boil and the subsequent bleach as described, it is possible to 'obtain from "unbleached soda and sulphate pulps products of very .good colour having an alpha-cellulose content of about '85- 89% and a pentosan content of about 7-9%. This pentosan content can easily be reduced, for example to about l-3 by the treatment with alkali of concentration above 8%, without causing any serious 'degradationof the 'cellulose molecule. For example, the bleached cellulose may be treated first with a caustic alkali solution 'ofconcen- 'tration about l0-20% at 20-25 C., or '(in another method) with a solution of concentration about 17-19% at about 4060 C., for a short time, e.g. between'about 45 and 90 minutes, and may them be boiled under atmospheric or preferably a higher pressure with a '0.252% caustic soda solution. Finally the purified cellulose may be given a very weak bleach with acid, neutral or alkaline hypochlorite.

V Cellulose obtained by the process of the present invention may be used for the manufacture of cellulose esters, especially esters of lower aliphatic acids such as cellulose acetate, propionate, butyrate and crotonate, and mixed esters of two or more of such acids, or for the manufacture of simple and mixed cellulose ethers soluble in Water, alkaline solutions or organic solvents, e.g. methy-l cellulose, ethyl cellulose, methyl ethyl cellulose, carboxymethyl cellulose, hydroxy-alkyl celluloses and benzyl cellulose. v

The invention is illustrated by the following examples.

Example I bleached cellulose was then treated in the cold with 10 phite was replaced by an equal weight of ammonium bisulphite.

Example 11 Another charge of the same pulp was boiled at 140 C. for 1 hour with 10 times its weight of an aqueous solution containing 0.35% of sodium bisulphite and 0.063% of sodium sulphite. The remainder of the treatment was carried out as described in Example I, and the final product had an alpha-cellulose content of 96.0% and a pentosan content of 1.39%.

Example III A charge of another sulphate pulp with about the same lig-nin content and a pentosan content of 10.3% was boiled at 140 C. for 2 hours with a 0.4% solution of sodium bisulphite, and then washed. The pulp Was then suspended in water at about 22 C. containing 0.04% of hydrochloric acid, and calcium hypochlorite added with stirring until the reaction ceased. The cellulose was then washed with Water, boiled for a short time with a 0.5% caustic solution, and washed free from alkali. The acid bleach was repeated under the same conditions except that the amount of bleach consumed was much less, and the cellulose was again washed, after which it was heated for 15 minutes to 70 C. with a 0.3% caustic soda solution, washed free from alkali, and bleached with a neutral hypochlorite. The bleached cellulose, after treatment with alkali and a final bleach as described in Example I, had an alpha-cellulose content of 95.8% and a pentosan content of 1.46%.

Example IV Another charge of the same pulp as treated in Example III was boiled for 1 hour at 140 C. with an aqueous solution containing 0.32% of sodium bisulphite and 0.05% of free sulphur dioxide. The product was washed, given a two-stage bleach with neutral hypochlorite, further purified with alkali, and given a final bleach, all as described in Example I. It then had an alpha-cellulose content of 96.8% and a pentosan content of 1.23%.

The products of all the above examples, when acetylated by means of acetic anhydride in the presence of acetic acid using sulphuric acid as catalyst, yielded cellulose acetate of good viscosity and clarity.

It will be understood that in any of Examples I, II or IV a bleach with two acid stages and one neutral stage as described in Example III could be used, and conversely the bleach of Example III could have been replaced by a bleach in two neutral stages as described in Example I. While these particular methods of bleaching combine with the sulphite boil and the subsequent alkali treatment to give a specially good product, the *sulphite boil may be followed by other bleaching processes, especially processes making use of chlorine or chlorine compounds.

Having described our invention, what We desire to secure by Letters Patent is:

1. Process for the purification of crude cellulose selected from the group consisting of sulphate pulp and soda pulp, which comprises heating said crude cellulose to a temperature above 100 C. with 5 to 15 times its weight of an aqueous solution of an inorganic salt of sulphurous acid of concentration below 2% containing 0 to 0.25% of free sulphur dioxide for at least 30 minutes, then bleaching the cellulose, and subjecting the bleached cellulose to the action of a caustic alkali solution of concentration above 8% until its pentosan content is below 3%.

2. Process for the purification of crude cellulose selected from the group consisting of sulphate pulp and soda pulp, Which comprises heating said crude cellulose to a temperature of to C. with 5 to 15 times its weight of an aqueous solution of an inorganic salt of sulphurous acid of concentration of 0.25% to 1% containing 0 to 0.25% of free sulphur dioxide for 30 to 90 minutes, then bleaching the cellulose, and subjecting the bleached cellulose to the action of a caustic alkali solution of concentration above 8% until its pentosan content is below 3%.

3. Process according to claim 2, wherein the crude cellulose is heated with an aqueous solution of a salt of sulphurous acid selected from the group which consists of the sulphites and bisulphites of the alkali metals, the alkaline earth metals, magnesium and ammonium.

4. Process according to claim 3, wherein the cellulose is bleached in two stages with neutral hypochlorite solumom.

5. Process according to claim 3, wherein the cellulose is bleached in two stages with acid hypochlorite solution followed by one stage with neutral hypochlorite solution.

6. Process according to claim 3, wherein the bleached cellulose is subjected to the action of a 10 to 20% caustic soda solution at 20 to 25 C. until its pentosan content is below 3%.

7. Process according to claim 3, wherein the bleached celluloseis subjected to the action of a 17 to 19% caustic soda solution at 40 to 60 C. until its pentosan content is below 3%, and is then boiled with a 0.25% to 2% caustic sod-a solution.

References Cited in the file of this patent UNITED STATES PATENTS 227,464 Wilson May 11, 1880 1,805,799 Benson May 19, 1931 1,809,286 Richter June 9, 1931 1,848,661 Richter Mar. 8, 1932 1,941,154 Richter Dec. 26, 1933 1,970,065 Tomlinson -Q. Aug. 14, 1934 2,054,854 Dreyfus Sept.'22, 1936 2,226,356 McCarthy et al. Dec. 24, 1940 2,239,606 Hill et al Apr. 22, 1941 2,380,706 Richter .July 31, 1945 2,564,028 Parrett Aug. 14,1951 2,645,576 Bate July 14, 1953 2,686,102 Hawkes et al. Aug. 10, 1954 'OTHER REFERENCES 7 7 Cellulose and Cellulose Derivatives by Ott, pages 500, 510-514, 813, 814, published by Interscience Publishers, Inc., New York (1943).

Meller: Paper Trade I Feb. 27, 1947, pages 104, 106.

Dennis: Paper Trade 1., Oct. 3, 1952, page 32.

Khutorshchekov: I Textile Institute, July 1944, p. 295. 

1. PROCESS FOR THE PURIFICATION OF CRUDE CELLULOSE SELECTED FROM THE GROUP CONSISTING OF SULPHATE PULP AND SODA PULP, WHICH COMPRISES HEATING SAID CRUDE CELLULOSE TO A TEMPERATURE ABOVE 100*C. WITH 5 TO 15 TIMES ITS WEIGHT OF AN AQUEOUS SOLUTION OF AN INORGANIC SALT OF SULPHUROUS ACID OF CONCENTRATION BELOW 2% CONTAINING 0 TO 0.25% OF FREE SULPHUR DIOXIDE FOR AT LEAST 30 MINUTES, THEN BLEACHING THE CELLULOSE, AND SUBJECTING THE BLEACHED CELLULOSE TO THE ACTION OF A CAUSTIC ALKALI SOLUTION OF CONCENTRATION ABOVE 8% UNTIL ITS PENTOSAN CONTENT IS BELOW 3%. 